Generally, a nuclear reactor means an apparatus which controls a chain reaction to emit a large amount of mass defect energy which is instantly emitted as a result of a chain nuclear fission reaction so as to use heat energy generated from nuclear fission as power. To secure safety and reliability of an atomic power plant, there is a need to periodically test a pipe or a nuclear reactor vessel. The test is performed by ultrasonic testing, and the like which mainly uses an ultrasonic wave. A testing system and a maintenance system of the nuclear reactor vessel are installed using a fixed platform, in which the fixed platform is fixedly mounted to the nuclear reactor vessel by a multi-axis manipulator manner.
However, the fixed platform needs to include a separate bracket for adjusting a length when the size of the nuclear reactor vessel is increased. In addition, the fixed platform may not change the multi-axis manipulator which is attached to a lower end thereof, such that the number of nuclear reactors which may be tested is limited and the fixed platform may hardly be decomposed/assembled and transferred/installed.